1. Field of the Invention
The present invention relates to a solid fuel boiler and a method of operating a combustion apparatus.
2. Description of the Related Art
For a solid fuel boiler, there have been demands for combustion at a high efficiency and for reduction of NOx and CO from environmental problems. To meet these demands, methods have been used such as combustion at a low air ratio, a two-stage combustion method, an exhaust gas re-circulation, and the use of a low NOx burner.
In the two-stage combustion method, combustion air is supplied from the burner and air inlet ports (hereinafter referred to as after air ports) disposed on the downstream side of the burner. An air amount in the burner is reduced, and thus, a reducing region in which oxygen is insufficient is formed in a furnace so as to reduce NOx. Furthermore, air is supplied from the after air ports so as to reduce unburned carbon.
In a method of recirculating exhaust gas, a part of the exhaust gas exhausted from the furnace is introduced into the furnace via exhaust gas ports disposed in the furnace on an upstream side of a burner stage or on a downstream side of the after air ports. Since the exhaust gas is recirculated into the furnace, a flow volume of gas flowing through the furnace is increased, and a heat absorption ratio is adjusted in a heat exchanger (water pipe) disposed on a furnace wall, and a heat exchanger disposed in a heat recovery area connected to an outlet of the furnace. Accordingly, steam is stably produced at a higher temperature and pressure, and it is possible to operate the boiler with high efficiency.
In JP-A-2000-46304, a technique is disclosed in which a part of combustion exhaust gas is recirculated to the furnace in order to reduce a thermal NOx concentration.
In this related art, a supply port of the combustion exhaust gas, having an annular section, is disposed in a wind box so as to surround a burner throat, a secondary air supply port and a tertiary air supply port. When such an annular supply port is disposed, an initial flame (having a temperature of about 1000° C.) in the vicinity of the throat of the burner is mixed with the exhaust gas, and the flame sometimes becomes unstable. As a result of the instability of the combustion of the initial flame, fuel NOx cannot be decreased sufficiently. Especially, when air spouted via the air nozzle of the burner is swirled, the initial flame in the vicinity of the burner throat is remarkably mixed with recirculation gas.
Moreover, as disclosed in JP-A-3-95302, there is also a method of supplying the recirculation gas in the vicinity of a bottom of the furnace. However, there is a possibility that the flame is blown off, and stable combustion cannot be performed.
As described above, the decrease of the flame temperature is a problem in a portion of the furnace having a high thermal load. When a maximum temperature of the flame is suppressed, it is possible to suppress ash stick troubles caused by melting or softening of ash on a wall surface, and generation of nitrogen oxide (thermal NOx). When stable combustion can be performed in the portion of the furnace having the low thermal load (corresponding to the initial flame whose temperature is about 1000° C.), fuel NOx and unburned carbon can be reduced.